1. Field of Invention
The present invention relates to an acoustic model creation method as well as an acoustic model creation apparatus which creates a Continuous Mixture Density HMM (Hidden Markov Model) as an acoustic model, and a speech recognition apparatus which employs the acoustic model created by the method or apparatus.
2. Description of Related Art
In speech recognition, there has been generally used a method wherein an HMM in phoneme or syllable units is used as an acoustic model, the phoneme HMMs or syllable HMMs are connected, and a speech in units of words or clauses is recognized.
With the related-art phoneme HMM or syllable HMM, however, the fluctuation of a phonemic environment in an interval longer than a syllable is difficult to be expressed, resulting in the problem that a speech recognition rate is impaired.
An undefined-length acoustic model which makes the unit of the HMM longer than a phoneme and expresses several phonemes in order to address the problem, has been known as stated in JP-A-8-123477 entitled “Undefined-length Acoustic Model Creation Apparatus and Speech Recognition Apparatus”.